In order to meet the need of reducing the cost and size of electronic equipment, an effort of reducing the size of a filter as a circuit constituent element is being made continuously without changing its way. Strict requirements on both of the size and cost of constituent parts are imposed on consumer electronic equipment such as a cellular phone, miniature radio, etc. A circuit contained in such electronic equipment uses filters which must be tuned precisely to predetermined frequencies. Accordingly, an effort of supplying inexpensive and compact filters is being continuously made at every moment.
Devices using the piezoelectric phenomenon have been used in various fields. In the progress of miniaturization and power saving of portable equipment such as a cellular phone, etc., the application field of surface acoustic wave (SAW) devices as RF filters or IF filters used for the above equipment is being enlarged. Enhancement of the design and producing technologies of SAW filters have satisfied user's strict requirements to specifications. However, as the frequencies being used are shifted to a higher frequency band, the enhancement of the characteristics is approaching to its upper limit, so that great technical innovation has been required for both of the microstructure of electrodes to be formed and securement of stable output.
Further, a thin film acoustic resonator using the thickness vibration of piezoelectric thin film, that is, a thin film bulk acoustic resonator or a film bulk acoustic resonator (hereinafter referred to as “FBAR”) is constructed by forming thin film mainly composed of piezoelectric element and electrodes for driving the piezoelectric thin film on a thin support film formed on a substrate. FBAR thus constructed can perform fundamental resonance in gigahertz band. If a filter is constructed by FBAR, the filter could be designed in a remarkably compact size, and also it can be operated with low loss and in a broad band. In addition, it can be manufactured integrally with a semiconductor integrated circuit. Therefore, it is expected that FBAR will be applied to future ultraminiature portable equipments.
A simple construction of the thin film bulk acoustic resonator has such a sandwich structure that a piezoelectric (PZ) thin film material layer is sandwiched between two metal electrodes. The sandwich structure is supported by a bridge structure in which the peripheral portion thereof is supported and the center portion thereof is suspended in the air. When electric field is generated by applying a voltage across the two electrodes, the piezoelectric (PZ) thin film material converts some of the electrical energy to mechanical energy in the form of acoustic wave. The acoustic wave propagates in the same direction as the electric field, and reflects at the interface between the electrode and the air. In the following description, the piezoelectric element may be abbreviated to PZ.
When mechanical resonance is induced, the thin film bulk acoustic resonator serves as an electrical resonator due to the electrical energy/mechanical energy converting property of the PZ thin film material. Accordingly, the filter can be constructed by the thin film bulk acoustic resonator. The mechanical resonance of the thin film bulk acoustic resonator is induced at the frequency at which the thickness of the material through which the acoustic wave propagates is equal to a half of the wavelength of the acoustic wave. The frequency of the acoustic wave is equal to the frequency of an electrical signal applied to the electrodes. The velocity of the acoustic wave is smaller than the velocity of light by 5 to 6 figures, and thus the resonator achieved can be made extremely compact. Therefore, the resonator to be used in the GHz band can be designed in the structure having a plane size less than 200 micrometers and a thickness less than several micrometers.
In the thin film bulk acoustic resonator and stacked thin film acoustic resonators having a plurality of the sandwich structures as described above, that is, stacked thin film bulk acoustic wave resonators and filters (hereinafter referred to as “SBAR”), the center portion of the sandwich structure comprises piezoelectric thin film which is manufactured to have a thickness of about 1 to 2 μm by the sputtering method. The upper and lower electrodes act as electrical leads and are disposed so as to sandwich the piezoelectric thin film therebetween to give electrical field penetrating through the piezoelectric thin film. The piezoelectric thin film converts a part of the electric field energy to mechanical energy. In response to the applied electric field energy which varies with time, the time-varying (stress/strain) energy is formed.
A piezoelectric thin film element applied to a resonator, a filter or the like which uses such elastic wave is manufactured as follows.
By using various thin film forming methods, a base film comprising a dielectric thin film, a conductive thin film or a stacked film of the dielectric thin film and the conductive thin film is formed on the surface of a single crystal semiconductor substrate of silicon or the like or on the surface of a substrate constructed by forming polycrystalline diamond film or film of isoelastic metal such as elinvar or the like on silicon wafer or the like. Further, piezoelectric thin film is formed on the base film, and a desired upper structure is formed. After each film is formed or after all the films are formed, each film is subjected physical processing or chemical processing to perform micro-fabrication and patterning.
In order to operate FBAR or SBAR as a thin film bulk acoustic resonator, the sandwich structure containing the piezoelectric thin film must be supported by the bridge structure so that the air/crystal interface is formed to confine the acoustic wave in the sandwich structure. The sandwich structure is generally constructed by stacking a lower electrode, a piezoelectric layer and an upper electrode in this order. Accordingly, the air/crystal interface has already existed at the upper side of the sandwich structure. In addition, the air/crystal interface must be also formed at the lower side of the sandwich structure. In order to achieve the air/crystal interface at the lower side of the sandwich structure, the following methods have been hitherto used.
According to a first method, as disclosed in JP(A)-58-153412 or JP(A)-60-142607 for example, etching away a part of wafer which constitutes a substrate is used. In the case where the substrate is composed of silicon, a part of the silicon substrate is etched away from the back side thereof by using heated KOH water solution to form a hole. That is, after a base film, a lower electrode, a piezoelectric thin film and an upper electrode are formed on the-upper surface of the substrate, a portion of the substrate located below a portion which will act as an oscillating portion is removed from the lower surface side of the substrate, thereby achieving a resonator having such a shape that the edge of the sandwich structure is supported by the silicon substrate at a portion surrounding the hole at the front surface side of the silicon substrate. However, the hole thus formed so as to penetrate through the wafer makes the wafer very fragile and easily breakable. Further, if wet etching using KOH is carried out at an etching inclination angle of 54.7 degrees with respect to the surface of the substrate, it would be difficult to increase the achievement amount of final products, that is, to increase the yield of FBAR/SBAR on wafer.
For example, the sandwich structure having a lateral dimension of about 150 μm×150 μm (plane size) which is formed on silicon wafer having a thickness of 250 μm needs a back-side etching hole of about 450 μm×450 μm. Accordingly, about only one ninth of wafer can be used for manufacturing. After a portion of the substrate located below the oscillation portion of the piezoelectric thin film is removed by anisotropic etching to form a floating structure, the wafer is separated every element to achieve thin film bulk acoustic resonators (which are also called as piezoelectric thin film resonators) corresponding to the PZ thin film elements.
According to a second method of providing the air/crystal interface below the sandwich structure, as disclosed in JP(A)-2-13109, an air bridge type FBAR element is formed. Normally, a sacrificial layer is formed, and then a sandwich structure is formed on the sacrificial layer. The sacrificial layer is removed at the end of the process or about the end of the process. The overall processing is carried out at the front surface side of wafer, and thus this method needs no positioning at the both sides of the wafer and no large-area back-side opening.
JP(A)-2000-69594 discloses the construction of an air bridge type FBAR/SBAR and a method of manufacturing the same using phospho-silicate glass (PSG) as a sacrificial layer. In this publication, a PSG layer is deposited on silicon wafer. PSG is deposited at a temperature of about 450° C. or less by using silane and P2O2 to form soft-glass-like substance containing a phosphorus content of about 8%. PSG can be deposited at a relatively low temperature, and it can be etched at a very high rate with dilute H2O:HF water solution.
It is described in this publication that variation of RMS (root mean square) of the height indicating the surface roughness of the PSG sacrificial layer is less than 0.5 μm, however, there is no description on RMS variation of an order of 0.1 μm or less. The 0.1 μm-order RMS variation is very roughly uneven at the atomic level. An FBAR/SBAR type thin film bulk acoustic resonator needs a piezoelectric material whose crystal is grown as prismatic crystal vertical to the plane of the electrode.
In JP(A)-2000-69594, it is described that a conductive sheet parallel to the surface of the PSG layer is formed, and the RMS variation of the height of the conductive sheet is less than 2 μm, however, there is no specific description on the RMS variation of an order of 0.1 μm or less. The 0.1 μm-order RMS variation is an insufficient surface roughness for the surface on which piezoelectric thin film for a thin film bulk acoustic resonator is formed. Various attempts to grow the piezoelectric thin film have been made. However, the crystal is grown in various directions due to the effect of various unevenness on the rough surface, so that the crystal quality of the piezoelectric thin film achieved is not sufficient.
There is a method of providing a proper solid acoustic mirror in place of provision of the air/crystal interface as described above. According to this method, as disclosed in JP(A)-6-295181, a large acoustic impedance comprising an acoustic Bragg reflection mirror is formed below the sandwich structure. The Bragg reflection mirror is formed by alternately stacking layers of high and low acoustic impedance materials. The thickness of each layer is fixed to one fourth of the wavelength of the resonant frequency. A sufficient number of layers enables the effective impedance at the interface between the piezoelectric element and the electrode to be still higher than the acoustic impedance of the element. Accordingly, the acoustic wave in the piezoelectric element can be confined effectively. An acoustic resonator achieved according to this method is called as solid acoustic mirror mounted resonator (SMR) because there is no cavity below the sandwich structure.
This method can avoid the problem of the first and second methods that there is formed such a film that the peripheral portion thereof is fixed and the center portion thereof is freely vibrated. However, this method has many problems. That is, since a metal layer forms a parasitic capacitor which degrades the electrical performance of the filter, it cannot be used as the layer of the Bragg reflection mirror. Therefore, selection of materials usable for the Bragg reflection mirror is restricted. The difference in acoustic impedance between layers formed of available materials is not large. Accordingly, a number of layers are needed to confine the acoustic wave. Further, in this method, stress applied to each layer must be controlled with high precision, and thus the manufacturing process is complicated.
Further, it is difficult to form a viahole penetrating a large number of layers such as the number of 10 to 14, and thus the acoustic resonators achieved according to this method are unfavorable to integration with other active elements. Further, according to-examples which have been ever reported, the acoustic resonators achieved according to this method have effective coupling coefficients still lower than those of the acoustic resonators having air bridges. As a result, the filter based on SMR has a narrower effective band width than the filter using the air bridge type acoustic resonator.
As described above, in the thin film bulk acoustic resonator, (stress/strain) energy that varies with time in response to time-varying applied electric field energy is formed in the sandwich structure. Accordingly, when the adhesion force between the substrate and the lower electrode of the sandwich structure is low, the substrate and the sandwich structure are exfoliated from each other, so that durability is lowered, that is, the lifetime of the thin film bulk acoustic resonator is shortened.
In JP(A)-2000-69594, etc., Mo is described as proper electrode material. However, there is no specific description on enhancement of adhesion force to silicon wafer, etc. serving as a substrate.
For example, JP(A)-2-309708 discloses that a lower electrode layer comprising two layers of Au/Ti or the like is used. In this case, the Ti layer exists as a layer for increasing the adhesion force between the Au layer and the substrate. That is, the Ti adhesion layer is not an indispensable electrode layer from the viewpoint of the original operation of the thin film bulk acoustic resonator. However, when no Ti adhesion layer is formed and only the Au electrode layer is solely formed, the adhesion force between the substrate and the Au electrode layer is insufficient, and occurrence of exfoliation, etc. remarkably reduces the durability of the thin film bulk acoustic resonator under operation.
In the thin film bulk acoustic resonators as described above, there exist not only required longitudinal vibration propagating in the direction vertical to the electrode surface, but also lateral vibration propagating in the direction parallel to the electrode surface. The lateral vibration contains a component that causes “spurious” in the required vibration of the thin film bulk acoustic resonator to degrade the characteristics of the resonator.
An object of the present invention is to provide FBAR/SBAR having improved performance.
Another object of the present invention is to provide a high-performance FBAR/SBAR that is excellent in electromechanical coupling coefficient, acoustic quality factor (Q-value), temperature characteristic, etc. by improving the crystal quality of piezoelectric (PZ) thin film.
Another object of the present invention is to provide a high-performance FBAR/SBAR that is excellent in electromechanical coupling coefficient, acoustic quality factor (Q-value), temperature characteristic, etc. by devising the shape of an upper electrode.
Another object of the present invention is to provide a high-performance FBAR/SBAR in which particularly spurious resonance is reduced.
Another object of the present invention is to improve durability of FBAR/SBAR and thus improve the lifetime thereof by increasing the adhesion force (bonding strength) between a lower electrode layer and a substrate.
Another object of the present invention is to provide a high-performance FBAR/SBAR that is excellent in electromechanical coupling coefficient, acoustic quality factor (Q-value), etc. by increasing the adhesion force between the lower electrode layer and the substrate and enabling formation of piezoelectric thin film having excellent crystal quality and orientation on the lower electrode layer.
As the piezoelectric materials for piezoelectric thin film elements are used aluminum nitride (AlN), zinc oxide (ZnO), cadmium sulfide (CdS), lead titanate [PT] (PbTiO3), lead titanate zirconate [PZT] (Pb(Zr,Ti)O3), etc. Particularly, AlN has higher propagation speed of elastic wave, and it is suitably used as piezoelectric material of piezoelectric thin film resonator for a thin film bulk acoustic resonator or filter which operate in a high-frequency band.
FBAR achieves resonance by elastic wave propagating in the thin film, so that not only the vibration characteristic of the piezoelectric thin film, but also the vibration characteristics of the electrode layer and the base film greatly affect the resonance characteristic of FBAR. Various attempts have been hitherto made to apply the AlN thin film to FBAR. However, there have not yet achieved any thin film bulk acoustic resonator and any thin film filter which can exhibit sufficient performance in GHz band. Accordingly, there has been strongly required a piezoelectric thin film resonator, that is, a thin film bulk acoustic resonator in which the electromechanical coupling coefficient, acoustic quality factor and the temperature stability of the resonant frequency of a vibration portion containing not only the AlN thin film but also an electrode layer and a base film are improved.
Therefore, the present invention has an object to provide a piezoelectric thin film resonator, that is, a thin film bulk acoustic resonator in which the temperature stability of the resonant frequency is improved without reducing the electromechanical coupling coefficient and acoustic quality factor by taking advantage of the characteristic of the AlN thin film with high propagation velocity of elastic wave.